(1) Field of the Invention
The present invention is directed generally towards a non-destructive method for detecting densities of components of a composite material (referred to herein as discontinuity densities and suspended discontinuity densities). More specifically it is directed toward determining micro-inclusion gradients and micro-inclusion gradient boundaries in micro-inclusion-impregnated materials utilizing sub-centimeter wavelength electromagnetic radiation. This method also detects micro-inclusion concentration levels in micro-inclusion-impregnated materials in a nondestructive, contactless fashion.
(2) Description of the Prior Art
Terahertz Time-Domain Spectroscopy (THz-TDS) was first developed in research institutions in the early 1990's. The technology began to see first commercial development later that decade with very large (the size of a refrigerator), very expensive apparatuses for niche research application. Over the following decade, THz-TDS systems became much smaller and operated with much greater dynamic range. For example, the current state of the art system is roughly the size of a suitcase and can image across a bandwidth of 0.03 to 3.5+THz (corresponding to a free space wavelength of about 1 cm to about 100 microns) with a dynamic range of about 100 dB.
Terahertz (THz) imaging combines aspects of optics, such as high resolution imaging, and radio-frequency electromagnetics to yield an imaging technology that can take sub-millimeter resolution pictures of objects hidden within and on the other side of materials. THz waves may not pass through all materials equally well; however, differences in material transparency allow for identification of one material hidden behind another, much like an electromagnetic version of ultrasound.
THz scanners are finding wider usage outside the research community. NASA has employed THz-TDS scanners to detect defects within the tiles on the outside of the Space Shuttle fuel tank. The Transportation Safety Administration (TSA) has employed THz scanners at airport security checkpoints nation-wide to detect concealed objects on people attempting to board planes.
Micro-inclusions are microscopic chunks of material manufactured for a wide variety of uses in research, medicine, consumer goods and various industries. Micro-inclusions are usually between 10 to 300 micrometers in diameter. They are used as lightweight filler in composite materials such as lightweight concrete. Micro-inclusions can impart the following qualities reduced weight, reduced thermal conductivity, and increased resistance to compressive stress that far exceeds that of other similar materials. These properties are exploited in high pressure environments where other similar materials would implode. Micro-inclusions having internal hollows create materials having different properties.
A material for testing can have a desired concentration of micro-inclusions. In making this material, a liquid (uncured) version of the material is put into a curing mold after being mixed with sufficient micro-inclusions to achieve the desired concentration. The buoyancy of the micro-inclusions in the curing liquid causes them to migrate towards the surface of the liquid opposite the pull of gravity while the material/micro-inclusion mixture cures. In other words, the liquid is denser than the micro-inclusions causing the liquid to sink while pushing the micro-inclusions closer to the upper surface. Thus, when the mixture is finished curing, there is a region in the resulting composite material at the lowest point of the material referenced to gravity where most of the micro-inclusions have floated away, and there is a region at the highest point of the material referenced to gravity having an excess of micro-inclusions. Naturally, there is a gradient in the micro-inclusion concentrations at the boundaries where the micro-inclusion concentration shifts due to the aforementioned gravitational/buoyancy effect. As a matter of quality control, it is desirable to maintain these concentrations within the desired limits.